Data communication is becoming increasingly important in today's society. For example, the popularity of the Internet has expanded demand for high quality, high speed access to a variety of services available on the Internet. Although infrastructure enabling such data communication allows transmission of data at high speeds over large distances, a barrier remains to providing high quality, high speed data communications. Communicating data between a home user or a business and a central office of a service provider requires either utilizing existing communication lines, such as phone lines, between the service provider and the home or business user, or adding additional infrastructure, usually at significant expense.
Digital subscriber line (DSL) technology addresses the problem of connecting the central office of a service provider to a home user or business by utilizing a frequency spectrum available on existing telephone lines that is not used for voice traffic. The frequencies at which data are transmitted and received according to DSL technology are higher than the frequency spectrum used for voice traffic on a standard telephone line. Asynchronous digital subscriber line (ADSL) technology is a particular form of DSL. With ADSL, a different data rate is used for upstream transmission of data than is used for downstream reception of data.
DSL technology utilizes a modem both at a customer's location and a remote location, such as a service provider's central office. These modems allow communication between the customer and the service provider according to a DSL protocol. DSL modems typically perform three functions: (1) voice and data separation, (2) channel separation, and (3) encoding and decoding. A DSL modem conventionally has a component referred to as a plain old telephone service(POTS) splitter, which divides the existing phone line to two bands, one for voice and one for data. Voice travels between zero and four kilohertz. The higher frequencies are used for data. Another component in a DSL modem, referred to as a channel separator, divides the data into two parts. In ADSL this includes a larger part for downstream data and a smaller part for upstream data.
DSL uses one of a number of technologies to encode and decode data on a single carrier frequency. These technologies include (1) quadrature amplitude modulation (QAM), (2) carrierless amplitude and phase (CAP) modulation, and (3) discrete multi-tone (DMT) modulation. Quadrature amplitude modulation is a modulation technique using variations in signal amplitude. The modulation technique encodes data as discrete phase plus amplitude changes of a carrier tone. Carrierless amplitude and phase modulation is a modulation technique that allows multiple bits of information to be represented by a single frequency cycle. Discrete multi-tone modulation is a modulation technique that separates the frequency range in which data is transmitted and received into a plurality of frequency bands, or channels, using a Fast Fourier Transform. One example of DMT utilizes 256 frequency bands and uses a quadrature amplitude modulation to modulate data in any of these frequency bands.
A problem that arises with the use of DSL technology is that increased distances between the two modems decreases the available bandwidth and therefore decreases transmission rates. Two reasons for this are: (1) the series inductance of the telephone wire increases with distance and a telephone therefore decreases available bandwidth, and (2) a telephone line generally runs parallel with other telephone wires, which provides a parallel capacitance that increases with distance, which further decreases bandwidth and transmission rates.
Conventional methods for testing a DSL modem are time consuming and costly and are therefore not effective methods for testing mass produced modems.